1. Field of the Invention
The present invention relates generally to epitaxial growth systems for production of semiconductor materials and devices, in particular. More specifically, the invention relates to the design of hydride vapor phase epitaxy (HVPE) growth systems and reactors, the design of internal components of HVPE growth systems and reactors, and HVPE-based processes for growth of group III-nitride materials and devices that can be used in optoelectronics as well as in high-power high-frequency electronics.
2. Prior Art
The development of GaN-based optoelectronics and power electronics has led to widespread research into the growth and applications of compounds of aluminum, gallium, indium, boron, and nitrogen (collectively, the “III-nitrides,” “group III-nitrides,” or “AlxInyGa1-x-yN” in which 0≦x+y≦1). Group III-nitride templates, junctions, heterojunctions, multi-layer structures, thick layers/films, and bulk materials are commonly grown epitaxially via chemical vapor deposition methods including, but not limited to, hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD, MOVPE, or OMVPE).
During these deposition processes, a group III-nitride is grown upon a substrate or template consisting of, but not limited to, sapphire, silicon, silicon carbide, magnesium aluminate spinel, gallium nitride, aluminum nitride, aluminum-gallium nitride alloys, indium nitride, and/or lithium aluminate. A template shall be understood to be a substrate of one of the preceding materials coated with a layer of group III-nitride material. For the purposes of this invention, the terms “substrate” and “template” will be used interchangeably, though one skilled in the art will recognize that slightly different growth chemistries are required to optimize a group III-nitride deposition process for each. The differences in required chemistries are independent, however, of the implementation of the invention as described below.
The choice of substrate material, the crystallographic orientation of the substrate, and the deposition method/chemistry strongly influence the crystalline and morphological quality of the group III-nitride grown upon the substrate/template.
During active growth of a III-nitride (the process by which group III-nitride material is added to the surface of a substrate), it is common for a substrate or template to be exposed to a “growth atmosphere” that contains both one or more group III precursors (including but not limited to gallium chloride, aluminum chloride, indium chloride, trimethyl gallium, triethylgallium, trimethyl aluminum, gallium hydride, and gallium metal) and an active nitrogen precursor (typically but not limited to ammonia, hydrazine, or dihydrazine). For the purposes of the present invention, the atmosphere or ambient conditions within a group III-nitride epitaxy chamber will be referred to as a “non-growth atmosphere” if either or both a group III precursor or a nitrogen precursor are absent from the gas phase chemistry in the vicinity of the substrates or templates on which the group III-nitride is grown.
Studies of the thermal and chemical stability of GaN and other III-nitride epilayers in various ambient gases have been undertaken that have demonstrated thermal instability of some of the III-nitrides in common growth ambient environments (see M. A. Mastro, O. M. Kryliouk, M. D. Reed, T. J. Anderson, A. Davydov, and A. Shapiro, Thermal Stability of MOCVD and HVPE GaN Layers in H2, HCl, NH3 and N2, Phys. Stat. Sol. (a) 188 (2001) 467-471 and M. A. Mastro, O. M. Kryliouk, T. J. Anderson, A. Davydov, A. Shapiro, Influence of polarity on GaN thermal stability, Journal of Crystal Growth 274 (2005) 38-46.). For example, on heating in N2, H2, NH3, and HCl, gallium nitride (GaN) can undergo dissociative sublimation or thermal decomposition accompanied in some instances by gallium droplet formation. In both cases the flatness and smoothness of the surface of the epilayer will be adversely affected, making it unusable for further device epitaxy.
It has been found that group III-nitride surfaces are generally more stable in non-growth atmospheres containing predominantly N2 and NH3 than in those containing principally H2, HCl, or Ar. The protective properties of nitrogen and ammonia are considered to be very useful when for some reason growth interruption is required and III-nitride surfaces are left exposed to the non-growth atmosphere (an ambient in which the III-nitride is not being actively deposited or grown). One skilled in the art of III-nitride epitaxy/crystal growth will recognize that III-nitride films and crystals are frequently exposed to such non-growth atmospheres during typical deposition/growth processes, such as during a waiting period for gas mixture homogenization at the beginning of an epitaxial run or during a slow cooling process at the end of the run. Growth interruptions also occur in the middle of the deposition cycles or runs for annealing to improve crystalline quality of the epilayer. Protection of the III-nitride surfaces is specifically important during the interruptions of an epitaxial process in which one of the components of a gas mixture may adversely influence surface morphology. An example of such an interruption can be found in the HVPE III-nitride deposition process employing HCl flow for the in-situ GaCl formation. The unreacted portion of the HCl flow is capable of etching unprotected surfaces of the substrate and epilayer. Indeed, such growth interruptions occur too frequently during a typical growth cycle for simple relocation of the epiwafers or crystals away from the growth zone of the reactor to adjacent so-called dwell zones to be sufficient or practical for ultimate preservation of the episurface. It is clear that there is a need for an effective means to protect heated III-nitride films, episurfaces, and crystals from decomposition. More preferably, there is a need for a means of providing protective gases through the dwell zone of the reactor to the III-nitride materials therein.
It was emphasized during the study of the first steps of the HVPE GaN growth on sapphire substrates that the results of the growth can be significantly influenced by the initial nucleation and nitridation conditions (see S. Gu, R. Zhang, Y. Shi, Y. Zheng, L. Zhang, F. Dwikusuma, T. F. Kuech, The impact of initial growth and substrate nitridation on thick GaN growth on sapphire by hydride vapor phase epitaxy, Journal of Crystal Growth 231 (2001) 342-351). To preserve the surface of the substrates or templates up to the growth temperature of 1100° C., an additional region, called a backflow tube, was introduced into a vertical HVPE reactor. The gas ambient within the backflow tube was chosen to be either pure N2 or N2+NH3 mixture depending on the choice of pregrowth treatment. The study confirmed that in the reactor with the backflow tube, improved initiation of the growth can be achieved. Apart from the improvement in the crystalline structure of GaN epilayers, their surface was generally smoother and had reduced density of surface pits, which was a common morphological feature of the grown epilayers. However, there was no specific consideration given to the geometry and position of the backflow tube inside the reactor that would help to prevent potential eddy backflows in the growth zone that could adversely affect III-nitride uniformity and quality. From the point of view of the present invention, when the position of the end of the backflow tube is too close to the growth zone or shape of this end coincides with the shape of the growth zone, gases flowing through the backflow tube will effectively block outflow of the reactive gasses from the growth zone. This blocking will negatively influence epitaxy in the growth zone and reduce all benefits of the backflow use to a minimum by unfavorably modifying the gas phase chemistry in the vicinity of the substrates.
In reference F. Dwikusuma and T. F. Kuech, X-ray photoelectron spectroscopic study on sapphire nitridation for GaN growth by hydride vapor phase epitaxy: Nitridation mechanism, Journal of Applied Physics 94 (2003) 5656-5664 the study of the sapphire nitridation in the backflow region of a vertical HVPE system under a NH3 and N2 ambient was described. It was mentioned that the backflow region allowed the sample to be heated to the temperature of 1100° C. under a countercurrent gas flow, protecting the sample from a gallium precursor stream. Nitridation was carried out by exposing the sapphire to a mixture of NH3 and N2 at a total flow rate of 2 slpm and a total pressure of 1 atm. The nominal reactor diameter near the sample that corresponded to the diameter of backflow region was 6 cm. The strict correspondence of the diameters and shapes of the backflow and growth regions inevitably leads to the generation of the gas vortexes in the growth zone resulting in irreproducible epitaxial condition. Indeed, the prior art has focused on the use of radially symmetrical backflow liners in vertical configurations that are of no use in horizontally configured flow paths and fail to address deleterious eddy current formation.
In a series of patents, the use of a backflow of ammonia for protection of the grown epilayers is claimed. For example, in U.S. Pat. No. 7,727,333 at a final step of the HVPE deposition of indium gallium nitride epilayer, the backflow of ammonia is provided into the reactor to prevent thermal decomposition of the grown epilayer. In the backflow, the substrate with the epilayer is allowed to cool down to the temperature at which decomposition is negligibly small even without ammonia.
In U.S. Pat. Nos. 6,656,272 and 7,670,435 to achieve sharp interlayer interfaces in multilayer structures the backflow gas sources and substrate movement within the growth zone are proposed. Once the growth of one sublayer is completed, the substrate is moved into the growth interruption zone where the backflow of an inert gas insures the interruption of the growth. While the substrate is in the growth interruption zone, the growth zone can be purged with the inert gas and active gas mixture including ammonia is reintroduced. After the growth mixture is uniformly distributed, the substrate is moved back into the growth zone. The Patents description does not include any specifications for optimal geometry of the growth interruption zone.
In U.S. Pat. No. 6,890,809 the backflow direction of argon and/or ammonia gases in the HVPE reactor is proposed to prevent undesirable growth during cooling the substrate with already grown epitaxial structure. In the preferred embodiment, the epitaxial structure comprises a GaN—AlGaN p-n heterojunction and p-type GaN capping layer helping to avoid surface oxidation of p-type AlGaN.
Along with a number of advantages that substantially increase flexibility of the HVPE process, the use of the backflow streams as disclosed in the prior art suffers a major drawback: it is a cause for induced eddy currents that can destroy the laminar pattern of the main gas flow and compromise stability and reproducibility of the group III-nitride growth process (see E. Richter, Ch. Hennig, M. Weyers, F. Habel, J. D. Tsay, W. Y. Liu, P. Brückner, F. Scholz, Yu. Makarov, A. Segal, J. Kaeppeler, Reactor and growth process optimization for growth of thick GaN layers on sapphire substrates by HVPE, Journal of Crystal Growth 277 (2005) 6-12).
The main objective of the present invention is the introduction of a backflow liner that decouples the active/main and counter/backflow gas streams in the growth reactor enriched with the gas counter-flow functionality. A further objective is to provide a backflow liner design that avoids deleterious eddy current formation as has been observed in the prior art and enables laminar gas flow in the vicinity of the growing group III-nitride films.
These objectives are achieved in the epitaxial growth reactor geometry provided herein comprising a main reactor element with an inserted backflow liner axially aligned to the adjacent growth liner and separated from it by the shaped opening that directed gas flow from the growth and backflow liners toward the reactor exhaust.